Return-type circuit is based on regenerative circuit. Regenerative circuit technology emerged in early-stage vacuum tube receivers first. Owing to the high price of vacuum tubes at that time, receiver designers began to seek paths for building the entire receiver with least number of vacuum tubes. An American Edwin Armstrong invented regenerative circuit, which made the implementation of single tube receiver possible. The working principle of regenerative circuit is: under the amplification and nonlinear actions of vacuum tube, an signal that contains amplified radio frequency signal and demodulated audio signal is output by radio frequency broadcast signal, and a part of the output signal is fed back to the input terminal of the vacuum tube again for amplification by means of positive feedback technology; in that way, oscillation of input signal occurs at the input terminal of the vacuum tube, and thereby the input radio frequency signal is “regenerated”. Since the oscillation tends to saturate, the ultimate amplitude of oscillation is fixed and the amplitude of output demodulated audio signal is also fixed for input signals with different intensities; the receiving volume is almost the same for radio stations with different intensities; therefore, the volume control process can be omitted. It is noted that the oscillation process should surely not be endless. In the circuit, the oscillation signal can be “reset” automatically at a short interval, so that oscillation is restarted and maintained according to the input signal, in order to track and regenerate the input signal at any time. In summary, regenerative circuit allows for operating in an intermittent non-steady state. In many cases, the function of receiver can be implemented with a single tube. Regenerative circuit breaks the provisions of gain-bandwidth product being constant in a first-order approximation system, and increases the equivalent Q value of the receiving circuit. The intermediate frequency amplification tube can be omitted, and the cost of receiver can be reduced; in addition, the power consumption is reduced, though reduction of power consumption was not a concern at that time. However, regenerative circuit in its early stage also faced some problems, for example, the circuit will amplify the noise infinitely when there is no signal input, causing hissing noise from the speaker; the emitted oscillating signal will interfere with other receivers; some circuit knowledge is required to accomplish adjustment of regenerative circuit, and the frequency selectivity is poor, etc. Regenerative circuit emerged in the times when all possible efforts were made to save tubes. Though the performance of regenerative circuit is barely satisfactory, the design concept of regenerative circuit has brought a unique idea to the circuit designers.
The positive feedback of regenerative circuit was the main cause for the drawbacks, but the full utilization of amplifier tube brought inspirations to the designers later. In the following several years, the prototype of return-type structure was put forth, i.e., the input high-frequency signal and the output demodulated intermediate frequency signal share the same amplifier stage. To prevent the occurrence of positive feedback, the radio frequency signal and intermediate frequency signal were isolated by means of high-pass and low-pass filter networks at the same time. However, the maximum frequency of short-wave audio broadcast at that time was only several MHz to tens of MHz, which differed only by 2 to 3 orders from the output audio. Therefore, the effect of isolation between radio frequency signal and output intermediate frequency signal was poor, and the receiving quality was poor due to mutual interference. With the emergence and development of transistor technology, the cost of tube was greatly reduced, and receivers with superheterodyne structure began to popularize; in comparison, the cost advantage of regenerative structure and return-type structure no longer existed. Therefore, regenerative structure and return-type structure were forgotten for a long time.
At present, radio frequency communication circuits usually have carrier frequency as high as 2-5 GHz and output intermediate frequency within a range of 2-10 MHz. The radio frequency signal and intermediate frequency signal can be isolated effectively by means of a simple first-order passive filter network. As portable communication means and wireless sensing networks are widely applied, how to reduce power consumption and increase battery life has become a purpose of the designers. The concept of radio frequency/intermediate frequency gain reuse of return-type circuit provides an approach for reduction of power consumption. In the present invention, the concepts related with the forgotten regenerative return-type circuit are merged into the design of integrated circuits, and thereby a novel low-power return-type frequency mixer is built.